I just got finished replacing the buttstock to my AR ,and now Im hearing some sort of rattling, and I dont know what it is. Everything seems to be working properly, but its sort of annoying. Anything I might have done wrong?

The bore inside your buffer maybe machined to wide a the "anti-bounce" counter weights are shifting back and fourth. Do you have a friend that has a Carbine? If so, swap buffers and see it the noise goes away.
ls